This invention relates to polyvinyl butyral (PVB) sheet plasticized with fatty acid esters and more particularly with a fatty acid ester blend which provides the sheet with improved penetration resistance without significant loss in its resistance to stress cracking polycarbonate or polyacrylate when in contact therewith as an interlayer in a laminated glass assembly.
In recent years sociologists and psychologists have started a movement toward prisons without bars. Thick layers of impact resistant polycarbonate initially used in such applications were quite early determined to be inadequate since the polycarbonate could be burned or scratched or dissolved or fractured when heated or abused. Moreover, when exposed to the elements the polycarbonate turned yellow, lost its strength and was susceptable to crazing. Based on this early work a need was defined for a material for security applications which was strong, chemically impervious, and scratch and fracture resistant over a wide range of ambient conditions.
Thereafter laminates of one or more layers of glass with one or more layers of polycarbonate were proposed which in use were mounted with the glass layer on the outside exposed to the elements and the polycarbonate either forming the innermost surface or positioned inboard of a glass layer forming such innermost surface. However, with these laminates an interlayer between the glass and polycarbonate is necessary since polycarbonate will not adequately adhere to glass.
Thermoplastic polyurethane layers have been used to laminate polycarbonate to glass but polyurethanes are costly and usually require treatment of the glass with a primer coating before lamination. Moreover, such polyurethanes are difficult to process into laminates because their low melting temperatures dictate the need for a batch type deairing operation with long exposure to vacuum for complete air removal. Commercially available grades of PVB sheet are not acceptable as an interlayer to adhere polycarbonate to glass because the plasticizer in the PVB crazes the polycarbonate. To solve this a special barrier coating, as disclosed in U.S. Pat. No. 4,243,719, was proposed between the polycarbonate and PVB to keep the plasticizer in the PVB away from the polycarbonate. The PVB is still necessary since the coating will not adhere to the glass and the PVB assists in absorbing energy on impacting. Such a coating must be pinhole-free for obvious reasons and moreover is costly in that it represents an additional layer in an already multilayered laminate which could be avoided if a plasticizer for PVB were available which did not craze the polycarbonate.
Sulfonamide and phosphate plasticizers for PVB which do not attack polycarbonate are disclosed in U.S. Pat. Nos. 3,539,442 and 3,406,086 respectively but PVB containing such plasticizers can only be difficultly extruded in forming sheet since they tend to decompose at extrusion conditions and degrade the PVB.
Certain esters which in natural form can be the major constituents of castor oil are proposed in U.S. Pat. No. 4,128,694 as plasticizers for PVB which neither craze polycarbonate or polyacrylate nor deteriorate during extrusion of plasticized PVB. However, such interlayers exhibit relatively high glass transition temperatures (Tg) at the usual plasticizer loadings and marginal peel adhesion to polycarbonate which means the impact properties at low performance temperatures are also marginal. This has precluded the use of such interlayers in security glass applications exposed to cold climates.
A need, therefore, exists in the art for a plasticized PVB sheet having the properties of those disclosed in U.S. Pat. No. 4,128,694, but which has a low Tg and higher peel adhesion and therefore improved low temperature performance properties when present as a functional PVB interlayer in a security glass laminate. Higher peel adhesion (i.e. equal to or greater than 10 newtons per cm) is needed to maintain laminate integrity during and after laminate impacting. However, very high peel adhesion (i.e. equal to greater than 30 newtons per cm) can produce laminate failure due to breakthrough when the laminate is impacted. On the other hand, very low peel adhesion (i.e. less than 10 newtons per cm) can produce delamination on impacting, e.g. along the laminate edges.